The present invention relates to a four wheel drive system which controls the distribution of a driving torque (or force) between front wheels and rear wheels.
One conventional example of a four wheel drive system is disclosed in Japanese patent provisional publication No. 58-26636. In this conventional system, driving torque is transmitted directly to one of front wheel drive shaft and rear wheel drive shaft, and driving torque is transmitted through a hydraulic type transfer clutch to the other of the front wheel and rear wheel drive shafts. The transfer cluch is normally held in a slippingly engaged state to drive the vehicle in a near two wheel drive mode in which the torque is transmitted through the transfer clutch only slightly. When a slip occures betweeen the front and rear wheels, the transfer clutch is brought into a fully engaged state to provide a complete four wheel drive mode. Thus, this control system is switched between the two wheel drive mode and the four wheel drive mode in a manner of the one-off control. Accordingly, the cornering characteristic of the vehicle shows a strong understeer tendency in the four wheel drive mode. In the case of the two wheel drive mode, this system is liable to cause a wheel slip duing a rapid start of the vehicle, and a spin during a turn of the vehicle.
Japanese patent applications Nos. 59-276048, 59-276049 and 60-35923 disclose four wheel drive systems designed to control the clutch engagement force of the transfer clutch in accordance with a speed difference between the front wheels and the rear wheels, and to vary a control characteristic in accordance with a condition such as a coefficient of friction of a road surface or a vehicle speed. However, these systems are arranged to determine a factor of proportionality of the control characteristic, independently of an accelerating condition of the vehicle. Therefore, these systems are liable to cause a wheel slip during a rapid start, and a spin during a turn with a rapid acceleration when the factor of proportionality is set equal to such a small value as to provide a driving torque distribution near to the two wheel drive mode. When the factor of proportionality is set equal to such a great value as to provide a driving torque distribution near to the four wheel drive mode, these systems are liable to increase the understeer tendency too much during a turn with a gradual acceleration.